This invention relates to a method and device for measuring defibrillation electrode resistance, and, more particularly, to such a method and device which employs a subthreshold current pulse, rather than a defibrillation shock, in connection with the measurement in order to avoid unnecessary stimulation of the patient's heart. The method may be used in either an implantable or an external defibrillating device.
The measurement of defibrillation electrode resistance is an important parameter in the field of defibrillation and especially during the process of implantation of implantable defibrillators. Knowledge of the defibrillation electrode resistance parameter has an additional advantage, subsequent to implantation and prior to the application of defibrillation therapy, following the detection of a tachyarrhythmia requiring therapy. Measured electrode resistance variations furnish useful information regarding electrode integrity and positioning. Comparisons may also be made to previous measurements to provide a parameter for the control of defibrillation threshold tracking. Another important advantage of the resistance measurement is to provide a means of maintaining a constant-energy defibrillation shock delivery system in implantable defibrillators.
A very useful application of defibrillation electrode resistance measurement at implantation is to verify the integrity and correct positioning of the defibrillation electrode connections prior to inducing tachycardia and subsequently delivering a defibrillation shock to revert the patient's rhythm. However, present methods are either unsuitable for implantable defibrillators, or they require a shock to be delivered, thus increasing the risk to the patient in the event that a successful reversion does not occur.
U.S. Pat. No. 4,574,810 to B. B. Lerman describes an external defibrillator which relies on low amplitude sinusoidal current to automatically ascertain the transthoracic resistance of a patient and then automatically apply a defibrillation shock according to the transthoracic resistance and an amperes per ohm factor. The use of a sinusoidal current, rather than a pulse, and the use of complicated and bulky circuitry make this device inappropriate for implantable defibrillators.
In an article entitled "Automated Impedance-Based Energy Adjustment for Defibrillation: Experimental Studies," by Kerber et al., in Circulation, Vol. 71, No. 1, pp. 136-140 (1985), it is shown how transthoracic impedance was predicted in advance of the first shock by passing a low level current between the defibrillator electrodes during the defibrillator charge cycle. This impedance prediction technique was described earlier by Geddes et al. in an article entitled "The Prediction of the Impedance of the Thorax to Defibrillating Current," in Medical Instrumentation, Vol. 10, No. 3, (May-June 1976). This method has an application to external devices, and is very complicated and difficult to administer in external defibrillators, with even greater difficulty in implantable devices. This method is time consuming and non-instantaneous and requires a continuous application of low level current.
The same method is further described in an article by Kerber et al., entitled "Advance Prediction of Transthoracic Impedance in Human Defibrillation and Cardioversion: Importance of Impedance in Determining the Success of Low-Energy Shocks," Circulation. Vol. 70, No. 2, pp. 303-308 (1984). The method requires a measurement circuit which uses a high frequency signal of 31 kHz passing through the patient via paddles. The high frequency signal flows during the defibrillator charge cycle. Although this method is unsuitable for implantable defibrillators and is unable to achieve the purposes of the presently disclosed invention, the article highlights to a certain degree the importance of accurately predicting transthoracic impedance, in advance, in the field of external defibrillation.
An article in IEEE Transactions on Biomedical Engineering, Vol. BME - 30, No. 6, pp. 364-367 (June 1983), by Savino et al., entitled "Transventricular Impedance During Fibrillation," refers to the two methods of impedance measurements currently used as being (1) the injecting of a low constant sinusoidal current, and (2) the determination of the time constant of an exponential defibrillatory discharge. This article also states that for all practical purposes, the impedance load between the electrodes and the heart may be considered as resistive.
By providing an instantaneous subthreshold measurement of defibrillation electrode resistance, the integrity of the electrode system in an implantable defibrillator can be established accurately prior to delivering a defibrillation shock, both at implantation and at the time of standard therapy to a patient having an implantable device. This is performed without any unnecessary stimulation of the patient's heart, thereby ensuring a high level of safety to the patient.
Also, with respect to another aspect of the invention, present constant energy defibrillation shock delivery systems, such as the system used by Cardiac Pacemaker Inc. (CPI) in their implantable defibrillator, maintain a constant energy defibrillation shock by varying the duration of the defibrillation shock. It is known mathematically that the amount of energy delivered is a function of the leading and trailing edge voltage of a truncated exponential wave-form. In the CPI implantable defibrillator, the trailing edge voltage is monitored, and when the desired trailing edge voltage is reached, the pulse is truncated.
The problem with this approach is that large shock durations result in non-optimal defibrillation thresholds. The defibrillation strength-duration curve during defibrillation exhibits a "U"-shape for a delivered energy. This is described by Tacker and Geddes at page 14 of "Electrical Defibrillation," published in 1980 by CRC Press of Boca Raton, Fla. As a result of this "U"-shaped curve, an optimum value for minimizing delivered energy can be chosen. Thus, any decreasing or increasing of the shock duration from this optimum value or "minimum energy point" will result in increased defibrillation thresholds.
Accordingly, it is desirable to use an improved constant energy algorithm which is based on the performing of a subthreshold defibrillation electrode resistance measurement prior to delivering a defibrillation shock.
It is, therefore, a primary object of the invention to ensure the integrity of defibrillation electrodes prior to delivering a defibrillation shock to a patient.
It is a further object of the invention to perform a defibrillation electrode resistance measurement using a subthreshold technique which will not affect the electrical timing of the heart and thereby reduces the risk to the patient.
It is another object of the invention to perform a simple and substantially instantaneous measurement of defibrillation electrode resistance.
It is a still further object of the invention to selectively obtain either a single measurement, such as prior to the delivery of a defibrillation shock, or to continually take measurements at preprogrammed intervals.
It is yet another object of the invention to provide for and control an improved constant-energy delivery system in an implantable defibrillator.
It is an additional object of the invention to provide a parameter for the control of defibrillation threshold tracking.
Further objects and advantages of this invention will become apparent as the following description proceeds.